ARM: tegra: only build __pinmux_nand() when it's needed

ARM: tegra: only build __pinmux_nand() when it's needed

__pinmux_nand() won't compile if PERIPH_ID_NDFLASH isn't defined.
Prevent this from causing build problems on newer SoCs without NAND
support (or without SW support for NAND yet), but preventing
compilation unless the function will actually be used, i.e. when
CONFIG_TEGRA_NAND is defined.

Signed-off-by: Tom Warren <twarren@nvidia.com>
[swarren, rewrote commit description, moved ifdef around whole function
rather than just body]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>